The present invention relates to motor stators created from packs of laminations. More particularly, the present invention relates to a motor stator created from packs of loose laminations where the center bore of the motor stator is drawn to size during the assembly of the motor.
Hermetic refrigeration compressors normally incorporate a compressor and an associated electrically driven motor within a hermetically sealed shell. The electric motor will generally include a rotor secured to a drive shaft journaled in the compressor housing or other suitable bearing means and a stator secured to the compressor housing by means of bolts extending through the stator core.
The stator core for these electric motors generally compress a plurality of stacked laminations bonded together by welding, varnishing or other means of bonding the individual laminations together. A set of windings is wound through the assembled stator core to produce the stator. The stator is mounted to a plurality of mounting pads or surfaces machined on the inner surface of the compressor housing. The stator is secured to the compressor housing using a plurality of bolts which are threadingly received by the compressor housing. The bonding of the laminations prior to assembly of the stator enables the correct alignment for the stator. The securing of the stator to the compressor housing results in a cantilever mounting arrangement for the stator in that only one end thereof is fastened to the compressor housing and the other end extends upwardly.
It is important to have the plane defined by the machined surfaces on the compressor housing perpendicular to the axis of the rotor when the rotor is installed. In order to accomplish this, it is necessary that the rotor accommodating bore of the stator core be perpendicular to the lower surface of the stator core itself, since it is this lower surface which is supported on the machined surfaces of the compressor housing. The proper positioning of the rotor accommodating bore of the stator core ensures that the air gap between the stator and rotor of the electric motor will be very uniform along the entire axial length of the electric motor. In many hermetic compressor applications, the rotor is supported at only one end thereof, and normally at the same end where the stator is connected to the compressor housing. Because of this cantilevered supporting arrangement for the rotor, although it is relatively easy to maintain an accurate air gap at the end nearest the bearing, normal flexing and deflection of the rotor at the opposite end will result in a wider variance in the air gap, taking into consideration normal machining and bearing tolerances. Accordingly, in order to minimize as much as possible the error in the air gap at the stator core furthest from the mounting surface of the compressor housing, it is necessary that the rotor accommodating bore be very accurately aligned perpendicular to the reference plane defined by the mounting surfaces on the compressor housing.
The plurality of laminations that make up the stator core can be a preassembled unit or they can be a plurality of loose individual laminations which are stacked up at the time of assembling the electric motor. When a preassembled unit is produced, the laminations are stacked together, aligned and then bonded together by welding, varnishing or by other means for bonding. The assembled units can have their rotor accommodating bore aligned during the assembling and securing operation or the rotor accommodating bore can be machined after assembly of the laminations. Once the stator core has been completed, the stator windings can be added to the core to produce the motor stator. The disadvantages to the preassembled stator cores include, but are not limited to, the relatively high costs associated with the alignment, the bonding and the subsequent machining of the rotor accommodating bore.
In order to improve the quality of the electric motors and reduce their costs, the industry has turned to the assembling of loose individual stator laminations into the stator cone and then adding the stator winding to this assembly of loose stator laminations. The problem to be solved, when assembling loose laminations, is to align the individual stator core laminations in such a manner that a very accurate size and perpendicularity of the rotor accommodating bore is achieved, not only at the bottom of the stack, but also at the top of the stack.
One prior art method for the assembling of stators having the loose individual stator laminations and the appropriate stator windings is to place the stator on the compressor body with the bolts extending through the loose stator laminations and loosely threadingly engaging the compressor body. An expanding mandrel is then placed through the center of the stator. The expanding mandrel is then expanded to form the final shape of the rotor accommodating bore. The expansion of the mandrel aligns the loose stack of stator laminations and the expansion of the mandrel will shear and/or crush any burrs which might be located within the bore. The individual bolts are then tightened to hold the stator laminations in place and to secure the stator.
While the above procedure has worked well for small fractional horse power electric motors, it does not work that well for the larger electric motors required for refrigeration compressors. The main reason for the failure of the above procedure is due to the large forces required to expand the mandrel to align all the laminations at once including the removal of any burrs. The large forces required for the above procedure create the possibility that the assembled stator will deform which results in a defective electric motor.
Thus, the continued development of the large motors needed for the refrigeration compressors includes the development of methods which will allow the use of the lower cost loose lamination electric motors for these large compressors.
The present invention provides the art with a method which makes it possible to utilize the lower cost loose lamination electric motors for the larger horse power electric motors required for the refrigeration industry. Prior to this unique method, the use of loose lamination electric motors was not possible for the larger compressors. The method comprises first assembling the loose lamination stator with the appropriate wiring to the compressor body using the bolts that extend through the stator core and then loosely threading the bolts into the compressor body. A solid mandrel is then pushed through this assembly. The solid mandrel is the same size or slightly smaller than the rotor accommodating bore formed in the individual laminations. This solid mandrel aligns the laminations one at a time while simultaneously repositioning the copper wire and the plastic liners and the solid mandrel pre-forms the rotor accommodating bore by shearing off and/or deforming any burrs that may be located within the bore. The solid mandrel is removed, a positioning pin is inserted into one of the open bolt holes and an expanding mandrel is inserted and expanded to form the final shape of the rotor accommodating bore. The expanding mandrel is then collapsed and re-expanded to secure the laminations in place while the bolts are tightened. The bolts hold the laminations in place to maintain the final bore shape. This two step alignment and forming process significantly reduces the loads of any single forming process thus allowing the use of the loose lamination stators for the higher horse power electric motors required for refrigeration compressors.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.